Sodium pyrithione (also called the sodium salt of 1-hydroxy-2-pyridinethione, sodium pyridine-2-thiol-N-oxide, or 2-pyridinethiol-1-oxide, Na salt) has excellent antimicrobial properties, and is typically employed as a biocide and preservative in functional fluids, such as metalworking fluids, lubricants, cosmetics and toiletries. Sodium pyrithione is a well-known commercial product commonly made by reacting 2-chloropyridine-N-oxide with NaSH and NaOH, as disclosed, for example, in U.S. Pat. No. 3,159,640.
Likewise, zinc pyrithione [also known as zinc pyridine-2-thiol-N-oxide or bis [1-hydroxy-2(H) pyridinethionato]-zinc] is an excellent antimicrobial additive. Zinc pyrithione may be made by reacting 1-hydroxy-2-pyridinethione or a soluble salt thereof with a zinc salt (e.g., ZnSO.sub.4) to form a zinc pyrithione precipitate, as disclosed, for example in U.S. Pat. No. 2,809,971. Zinc pyrithione has been employed as a broad-spectrum anti-microbial agent and preservative in metalworking fluids, plastics, paints, adhesives and cosmetics. Its principal uses are as an anti-dandruff agent in hair products, as a preservative in various cosmetics, and as an antifoulant in marine paints. The commercial use of zinc pyrithione in paints, adhesives, caulks and sealants is growing.
In the presence of ferric ion, sodium or zinc pyrithione-containing compositions tend to turn blue even though the ferric ion is present in mere trace amounts. This blue discoloration is undesirable for aesthetic reasons, as well as for functional reasons relating to unwanted color formulation.
More specifically, since the aesthetics of paints, adhesives, caulks and sealants normally require certain desirable colors, and since the formulators of such products go to great lengths to achieve specific color effects, any ingredient which causes the formulation to vary much from a desired white or colorless (i.e., "water white") hue may make the colorant formulators' task very difficult. More specifically, when attempting to utilize pyrithione as an antimicrobial agent in fully-formulated water-based paints, paint bases (i.e., the partially formulated paint before pigment addition), adhesives, caulks and sealants, an unwanted color in an additive can adversely affect the color of the formulated product. Thus, the discoloration typically adversely affects the desired color, producing an off-color product.
In addition to the aesthetics problems and colorant functioning problems, the blue coloration problem associated with the presence of ferric ion causes a performance problem in the sodium pyrithione-containing compositions. This performance problem when using sodium pyrithione results from the fact that the pyrithione tends to form a blue precipitate in the presence of ferric ion. The precipitate reduces the amount of available pyrithione throughout the composition, thereby diminishing the biocidal protection thereof.
In the past, various solutions to the blue discoloration problem have been proposed. By way of illustration, U.S. Pat. Nos. 4,957,658 and 4,818,436 disclose solutions to the above-discussed discoloration problem attributable to the presence of ferric ion plus pyrithione, in paints and functional fluids (e.g., metalworking fluids) respectively, by adding to the paint or functional fluid an alkali metal or alkaline earth metal salt of 1-hydroxyethane-1,1-diphosphonic acid. Although the '658 and '436 patents propose good solutions to the discoloration, these solutions are not always as cost effective or permanent as might be desired.
As another illustration, U.S. Pat. No. 4,161,526 discloses a white to cream yellow pyrithione, pyrithione salt or dipyrithione for application to skin or hair containing from about 0.01 percent to about 1 percent of the zinc salt of an organic carboxylic or inorganic acid, zinc hydroxide or zinc oxide, or a mixture thereof. The composition of the '526 patent is said to be effective in preventing or removing discoloration caused by formation of a colored pyrithione, pyrithione salt, or dipyrithione contaminant (said to be iron pyrithione) in the composition. Unfortunately, the '526 patent does not teach a solution to the discoloration problem in compositions unrelated to skin or hair care, and not containing iron pyrithione. Furthermore, the '526 patent teaches the use of higher levels of the zinc salt, zinc oxide or zinc hydroxide than otherwise might be desired.
New solutions to the blue and green discoloration problem in various aqueous compositions, particularly paints, adhesives, caulks and sealants which enables pyrithione to be utilized therein in the presence of iron or copper, without any resulting discoloration of the composition, and that is cheaper, longer lasting, and/or uses lower levels of additives than required by the above-discussed prior art, would be highly desired by the paint, adhesives, caulks and sealants manufacturing community. The present invention provides one such solution.